1. Field of the Invention
This invention relates to digital video signal processing and in particular to an apparatus and to a method for processing a digital video signal.
2. Description of the Prior Art
Video processing operations such as filtering and interpolation use the values of a plurality, for example a patch, of input pixels to form an output pixel value. For example, a simple 4.times.4 `area filter` generates the value of an output pixel by averaging the values of the input pixels in a 4.times.4 patch. At the edges of a picture, there is normally a sharp transition from, or into, blanking at black level. This is usually a highly aliased transition, which causes filters, interpolators and the like to `ring` and produce strange effects at the edge of the picture. Even if the profile of the edge transition is anti-aliased, for example with a transition over say 3 to 5 pixels, so as to try to prevent ringing, there will still be a spread of the edge effects which will erode the usable picture area.
The active acquisition area of a high-definition SMPTE 240M standard digital video frame is 1920 pixels (horizontally) by 1035 pixels (vertically), but in view of the above-mentioned problems some have recommended that the usable production and distribution area of the frame should be cropped so that the production and distribution area is reduced to, say, 1888 pixels by 1017 pixels. (Although embodiments of the invention will be described below in relation to one type of high-definition video signal, it should be noted that the invention is equally applicable to other high-definition and to conventional-definition signals.)